This invention relates, in general, to electrical signal filters which appropriately pass or reject a repetitive string of electrical pulses and, more specifically, to recursive radar filters suitable for filtering the clutter spectrum out of pulse-doppler radar echoes.
In pulse-doppler radar systems, it is necessary to utilize a filtering arrangement at the radar receiver to selectively pass or reject desirable and undesirable characteristics of the detected echo signal. Received pulses centered around the original transmitted frequency with relatively small frequency change are characteristic of unwanted echoes, or clutter. Consequently, it is desirable to filter out the clutter and preserve the other information contained in the echo pulses, such as the frequency shifted pulses returned from moving targets.
Various types of filters have been used in the past. Many of these filter arrangements used delay lines in order to help cancel out the clutter pulses. Because of the relatively long delay required, acoustical and optical delay devices were frequently used in the prior art. Advances in technology eventually permitted the use of analog electrical circuits and, most recently, radar clutter filters have been implemented using digital processing techniques.
In certain applications, it has been found advantageous to utilize a batch of pulses rather than process a continuous stream of pulses. When using such a batch of pulses, it is highly desirable to integrate as many of the returned pulses as possible in order to maintain a high sensitivity.
Unfortunately, prior art clutter filters with desirable filtering characteristics have required multiple delay circuits and recursive feedback to achieve the desired filtering. This arrangement of delay and feedback has made these filter devices susceptible to undesirable transient response and a ringing condition occurs frequently when large clutter signals are applied to the filter input. This condition can also occur when the repetition rate of the radar pulses changes. The transient response can swamp or mask the desired target echo until the transients have settled down.
One successful method for limiting the detrimental effects of transient response in recursive clutter filters employs the tapering of the input pulses. Although helpful for limiting transient response, the efficiency of the system is degraded because the extra pulses can only be used for transient dampening and not for target processing. Another method blocks or ignores some of the first pulses from the recursive filter until the filter is operating more in a steady state condition. Because of these inefficient methods of making recursive clutter filters usable, many radar systems have simply used feed-forward, non-recursive filters when short batches of pulses need to be filtered. Although feed-forward filters do not tend to have transient problems, the velocity response or filtering action of the filter is not as good as a recursive clutter filter.
Further background on recursive clutter filters can be found in Chapter 4 of "Introduction to Radar Systems" by Merrill I. Skolnik, published by McGraw Hill, Inc., (1970).
U.S. Pat. No. 4,345,252, entitled "Non-Linear Dynamic Filtering Device for the Angular Measuring of Noise in a Radar," describes a radar filter for use in enhancing the angular measurements of radar or sonar systems. While the device in the referenced patent employs the use of delay circuits and variable gain amplifiers, there are distinctive differences between the teachings in the referenced patent and the invention disclosed herein. In the referenced patent, the input signals are not in the form of repetitive pulses but are sinusoidal in nature. The variable gain is dependent upon the value of the analog input of one of the quantities used to determine the angular deviation of the system. In the invention disclosed herein, the gain of the amplifier circuits, as will be described later in detail, is determined by a time varying formula, that is, the period between successive pulses.
In view of the background and of the prior art, it is desirable and it is an object of this invention, to provide a radar clutter filter which exhibits excellent pass and reject characteristics on relatively short batches of pulses.